It is well known that a conductive resin is produced by kneading and dispersing a conductive filler in an electrically insulative resin for making the resin conductive for the purpose of prevention of static charge and the like. Examples of a material used as a conductive filler kneaded in a resin generally include ion-conductive organic surfactant, metal fiber and powder, conductive metal oxide powder, carbon black, carbon fiber and graphite powder. The material is melt-kneaded and dispersed in a resin to give a conductive resin composition, which is then molded to give a molded product with a volume resistance of 10−1 to 1012 Ω·cm.
If a flaky, whiskery or fibrous material with a large aspect ratio (length/outer diameter) is used as a conductive filler, even a relatively small amount thereof provides the conductivity to the resin. This is because a conductive filler with a larger aspect ratio forms inter-filler connection more effectively even if the same amount is used, resulting in imparting conductivity with a smaller amount.
A metal filler is, however, unsatisfactory in corrosion resistance and chemical resistance. Since an inorganic conductive filler is generally in a form of particle, it must be used in a large amount of more than 50% by weight to the total weight of a composition, which leads to the deterioration of physical properties of the resultant resin, and therefore, molding becomes more difficult. As carbon black, Ketjen Black (registered trademark, from Ketjen Black International Company) and acetylene black which form a conductive circuit having a chain structure can be utilized, so that it can impart high conductivity in an amount of 15% by weight or less. It is, however, difficult to control dispersion of these materials in a resin and special formulation and blending technique is required for achieving stable conductivity. Furthermore, even when adequate conductivity is achieved, not only the processability of the conductive resin composition is considerably deteriorated, but also physical properties such as tensile strength, bending strength and shock resistance are significantly deteriorated in comparison with original physical properties of the resin containing no conductive filler.
A conductive filler having a higher aspect ratio such as a flaky graphite powder and a whiskery carbon fiber must be also used in an amount of more than 15% by weight for imparting conductivity, which leads to deterioration in original properties of the resin, and thus, when a molding having a complex shape is to be formed, influence of non-uniformity and orientation of fibers becomes so significant that moldability and conductivity is deteriorated. Carbon particles or carbon fibers are easily detached from a molding surface (sloughing property), leading to problems such as environmental pollution and device damage in a semiconductor device process.
When carbon fibers are added in an equal weight, fibers having a smaller diameter are better in imparting conductivity because of easier formation of an inter-fiber conductive circuit network. Recently, hollow ultrafine carbon fibers having a smaller fiber diameter by two to three orders of magnitude than a conventional carbon fiber, so-called carbon nanotube, have been disclosed and it has been suggested to add it in a variety of resins and rubbers as a conductive filler (Patent Reference No. 1: Japanese Laid-Open publication No. H01-131251, Patent Reference No. 2: Japanese Laid-Open publication No. H03-74465, and Patent Reference No. 3: Japanese Laid-Open publication No. H02-235945), and it is believed to be an effective conductive filler eliminating the defects of the conventional conductive filler.
However, carbon nanotubes cannot be homogeneously dispersed in a resin and undispersed carbon nanotubes remain in the resin as an agglomerate, which causes unacceptable problems such as difficulty in spinning (yarn breakage), clogging of a filter in a discharge unit of a molding machine, deteriorated mechanical strength such as shock resistance of a molding and surface appearance. Therefore, special technique for composition formulation and blending and special surface modification treating are needed, including optimization of a resin molecular weight (Patent Reference No. 4: Japanese Laid-Open publication No. 2001-310994), addition of a modified resin, an elastomer and a compatibilizing agent (Patent Reference No. 5: Japanese Laid-Open publication No. 2007-231219, Patent Reference No. 6: Japanese Laid-Open publication No. 2004-230926, Patent Reference No. 7: Japanese Laid-Open publication No. 2007-169561, and Patent Reference No. 8: Japanese Laid-Open publication No. 2004-231745), and surface modification of a carbon nanotube (Patent Reference No. 9: Japanese Laid-Open publication No. 2004-323738), which limit the type of a resin and formulation, and the like.